Gear oil is present in the clutch cap in wet clutches or wet dual clutches; this means that in gears with such clutches, there is an additional drag moment that acts upon the gear input shafts. The drag moment depends on the viscosity of the gear oil. The viscosity of the gear oil, and thus the drag moment, increases greatly, especially at low temperatures as far as the customarily used gear oils are concerned, for example, in the so-called ATF (automatic transmission fluid) oils.
Such drag moments occur on account of low temperatures, for example, after protracted idle times in motor vehicles at low temperatures, and result in the fact that the dynamics of the gear will decrease in a subjectively perceptible fashion. That is expressed, for instance, in longer switching times. There are unknown measures to compensate for this effect that is brought about by the increased viscosities in clutch fluids.
DE 196 39 376 C1 discloses an automatically controlled clutch in the power train of a motor vehicle, where a regulating distance of a regulating unit is adapted for clutch actuation at high gear oil viscosities. Basically, the clutch is kept closed with an overpressure that is altered in analogy to the torque of the engine, that is to say, the pressure-applying spring elements are in each case so severely deformed that the frictional connection of the clutch will permit the transmission of a moment that will be placed by a limited measure above the particular engine moment. At low outside temperatures, this measure of overpressure is diminished in comparison to the normal state, as a result of which one can make sure that the clutch will be released quickly also at low temperature when it is recognized that the driver wishes to switch.
DE 198 23 772 A1 discloses a method for the compensation of increased viscosity of hydraulic fluid for the operation of an automated clutch. First of all, one determines a characteristic temperature, for example, an outside air temperature, cooling water temperature, suction air temperature, or some other temperature that is measured either directly or that is determined with the help of a mathematical model from other magnitudes. During movement of the transmitter piston in the clutch closing direction in a system consisting of transmitter and receiving cylinders, the comparatively viscous hydraulic fluid may possibly not flow quickly enough through a line between the transmitter cylinder and the receiver cylinder so that a pressure drop will build up in the working space of the transmitter cylinder. If the piston in the transmitter cylinder during its movement does not sweep over a shifting borehole, then there will be a change in the system volume, as a result of which the actuation accuracy of the clutch will suffer. As a remedy at low temperatures, movement speed of the transmitter piston in the clutch closing direction is changed toward the slower speeds. The slower speed suffices for the hydraulic fluid to flow through the line between the transmitter cylinder and the receiver cylinder so that no pressure drop will build up in the working chamber of the transmitter cylinder.
As an alternative, DE 198 23 772 A1 proposes that, at low temperatures when closing the clutch, one always runs over the shifting borehole of the transmitter cylinder so that the defined initial conditions will prevail during the next actuation cycle.
EP 0 479 464 B1 finally discloses a method for determining the viscosity of the gear oil where—instead of inserting a separate temperature probe for the measurement of a characteristic temperature—one records the gear sump lubricant viscosity from a slowdown of the rpm of the input shaft when the gear is on idle and the clutch is disengaged. The sump temperature, or a state that is called “cold/not cold,” is determined from the known lubricant characteristics and the slowdown rate of the shaft.